1. Field of the Invention
The present invention relates to diffractive optical elements, and more particularly to a diffractive optical element having such a grating structure that rays of light of a plurality of wavelengths or a specific wavelength concentrate onto diffraction light of a specific order (design order) of diffraction, and to an optical system having the diffractive optical element.
2. Description of Related Art
Heretofore, there have been known various methods of correcting chromatic aberration in an optical system. According to one of the known methods, two glass (lens) materials which differ in dispersion are combined with each other to be used for abating chromatic aberration.
According to another known method, chromatic aberration is abated by using a diffractive optical element having a diffracting function for an optical system which includes a refracting lens, as disclosed, for example, in the optical literature such as SPIE Vol. 1354 xe2x80x9cInternational Lens Design Conference (1990)xe2x80x9d and also in the specifications of Japanese Laid-Open Patent Applications No. HEI 4-213421 and No. HEI 6-324262 and U.S. Pat. No. 5,044,706.
This method has been developed by utilizing a physical phenomenon that the direction in which chromatic aberration arises in a ray of light of a certain wavelength with respect to a ray of light of a reference wavelength on a refractive surface becomes reverse to that on a diffractive surface.
Further, such a diffractive optical element can be provided with an effect of serving as an aspheric lens by varying the period of a periodic structure of its diffraction grating, so that aberrations can be abated advantageously.
Comparing refractive and diffractive surfaces in respect of a refracting action of rays of light, one ray of light remains one ray after refraction on a lens surface, whereas one ray of light is splint into rays of different orders when it is diffracted by a diffraction grating.
Therefore, in using a diffractive optical element for a lens system, it is necessary to design the grating structure in such a manner that light fluxes of a useful wavelength region concentrate onto diffraction light of a specific order (design order) of diffraction. With light fluxes concentrating onto diffraction light of the design order, in order to lower the luminous intensity of diffraction light of orders other than the design order, it becomes necessary that the diffraction efficiency of diffraction light of the design order is sufficiently high. Further, if there are some rays of light of diffraction orders other than the design order, these rays become flare light, because they are imaged in a place different from the imaging place of the rays of the design order.
For an optical system having a diffractive optical element, therefore, it is important to pay sufficient heed to the spectral distribution of the diffraction efficiency of diffraction light of the design order and also to the behavior of diffraction light of orders other than the design order.
FIG. 11 shows a case where a diffractive optical element 1 having one diffraction grating 4 formed on a base plate 2 is formed on a certain surface in an optical system. In this case, the diffraction efficiency for diffraction light of a specific order of diffraction is obtained as shown in FIG. 12, which shows in a graph the characteristic of the diffraction efficiency. In FIG. 12, the abscissa axis of the graph indicates wavelength and the ordinate axis indicates diffraction efficiency. The diffractive optical element 1 is designed to have the highest diffraction efficiency at the first order of diffraction (shown in a full line curve in FIG. 12) in the useful wavelength region.
In other words, the design diffraction order of this diffractive optical element is the first order. The graph of FIG. 12 also shows the diffraction efficiency of diffraction light obtained at diffraction orders near the design order, i.e., a zero order and a second order (1xc2x11).
As shown in FIG. 12, the diffraction efficiency at the design order becomes highest at a certain wavelength (540 nm) (hereinafter referred to as the design wavelength) and gradually lowers at other wavelengths. Such a lowering portion of the diffraction efficiency obtained at the design order becomes diffraction light of other orders, thereby appearing as flare light. Further, in a case where a plurality of diffractive optical elements are used, a drop in diffraction efficiency at wavelengths other than the design wavelength eventually causes a decrease in transmission factor.
The arrangement of lessening such a drop in diffraction efficiency is disclosed in Japanese Laid-Open Patent Applications No. HEI 9-127321, No. HEI 9-127322, etc. The diffractive optical element disclosed in Japanese Laid-Open Patent Application No. HEI 9-127321 is in a sectional shape formed by laminating two layers 4 and 5, as shown in FIG. 13.
The diffractive optical element disclosed in Japanese Laid-Open Patent Application No. HEI 9-127322 is of such a grating structure that three layers 4, 5 and 6 are laminated as shown in FIG. 14. As shown in FIG. 14, the layer 5, which is sandwiched between two diffraction grating surfaces 8 and 9 provided at the boundaries of the layers 4, 5 and 6, has a thickness which varies with portions thereof. In this diffractive optical element, each of the diffraction grating surfaces 8 and 9 is formed between two different materials. A high diffraction efficiency is attained by optimizing a difference in refractive power between the layer materials located in front and in rear of the boundary and the depth of the grating grooves.
Since the arrangements of the above-mentioned diffractive optical elements necessitate a wavelength characteristic of the difference in refractive power between the materials in front and in rear of each of grating areas to have desired values, it is impossible to have a larger difference in refractive power than in a case where a grating area has air on one side thereof instead of a layer material. As a result, their gratings must be arranged to have a relatively large grating thickness. In the case of the diffractive optical element disclosed in Japanese Laid-Open Patent Application No. HEI 9-127321, for example, the grating thickness is 10 xcexcm or thereabout.
In the case of the diffractive optical element disclosed in Japanese Laid-Open Patent Application No. HEI 9-127322, the number of layers of three different materials is increased to three and the number of gratings is increased to two. One of the two gratings measures at least 7 xcexcm in thickness, so that a considerably deep grating shape would be recognized.
In manufacturing diffractive optical elements, the above-stated grating shapes can be formed by cutting. A product thus obtained by cutting either may be used directly as a diffractive optical element or may be used as a mold for duplicating diffractive optical elements.
It is conceivable as a simplified manufacturing method to form an arcuate diffraction grating surface with a cutting tool edge 17 by rotating a base plate 2 as shown in FIG. 15. In this method, while the cutting tool edge 17 is moved in the direction of a grating pitch, the cutting process is carried out by varying a distance between the base plate 2 and the cutting tool edge 17 to obtain a desired shape of grating.
According to this manufacturing method, if the grating thickness is large as mentioned above, the amount of cutting by the cutting process increases to cause the cutting tool edge to be greatly abraded. As a result, the shape of the tool edge obtained at the commencement of cutting differs from its shape obtained at the end of cutting. Such abrasion causes the grating thickness at the point where the cutting comes to an end to become thinner than a desired value. In addition to this problem, since the cutting tool edge is rounded by the abrasion, the grating shape comes to deviate from a desired saw-tooth like shape.
Besides, since the grating thickness is thick with respect to the grating pitch, the slanting plane of grating slants steeper than the one-layer type conventional diffractive optical element. Therefore, the fore end of the cutting tool edge must be formed at a sharper angle than the slanting plane of grating. This necessitates use of a cutting tool edge at a sharper angle than a cutting tool edge for the one-layer type conventional diffractive optical element. The sharper angle of the cutting tool edge then causes the cutting tool edge to be more readily abraded.
The abrasion of the cutting tool edge may be abated by arranging its fore end part to have a duller angle. However, the duller tool edge angle necessitates the slanting plane of the saw-tooth-shaped (or serrated) grating to have a duller angle for preventing it from interfering with the cutting tool edge. The adoption of the laminated structure causes the grating thickness to be thicker and to have a steeper slanting plane than the grating of an ordinary one-layer type diffractive optical element. However, an attempt to moderate the angle of the slanting plane of the laminated structure causes a great increase in grating pitch. The usable range of such a diffractive optical element, therefore, would be limited by the increase in grating pitch.
It is an object of the invention to provide a diffractive optical element, or an optical system having the diffractive optical element, which is arranged to excel in workability of the diffractive optical element or in workability of a mold to be used for mass production of the diffractive optical element.
To attain the above object, in accordance with a mode of the invention, there is provided a diffractive optical element, which comprises a plurality of laminated diffraction grating surfaces, wherein each of the diffraction grating surfaces is formed to have a sufficiently small grating thickness as compared with a grating pitch thereof.
In accordance with another mode of the invention, there is provided a diffractive optical element, which comprises a plurality of laminated diffraction grating surfaces, wherein letting a grating pitch and a grating thickness of each of the diffraction grating surfaces be denoted by P and d, respectively, the following condition is satisfied:
d/P less than 1/6. 
In accordance with a further mode of the invention, there is provided a diffractive optical element, which comprises a plurality of laminated diffraction grating surfaces, wherein letting a grating thickness of each of the diffraction grating surfaces be denoted by d (xcexcm), the following condition is satisfied:
1 less than d less than 6. 
In accordance with a still further mode of the invention, there is provided a diffractive optical element, which comprises a plurality of laminated diffraction grating surfaces, wherein letting a grating pitch and a grating thickness of each of the diffraction grating surfaces be denoted by P (xcexcm) and d (xcexcm), respectively, the following conditions are satisfied:
d/P less than 1/6 
1 less than d less than 6. 
In accordance with the best mode of the invention, the diffraction grating surfaces are formed respectively with materials which differ from each other in dispersion, each of the diffraction grating surfaces is in a blazed shape with respect to a section of the grating pitch thereof, a useful wavelength region is an entire visible spectrum, and a diffraction efficiency of diffraction light of a specific order other than a zero order is high over the entire visible spectrum, including nearly 100%.
In accordance with a further mode of the invention, slant directions of gratings of the plurality of diffraction grating surfaces of the blazed shape are identical, and in accordance with a still further mode of the invention, a slant direction of a grating of at least one of the diffraction grating surfaces is opposite to that of the other diffraction grating surfaces.
In accordance with a further mode of the invention, the diffractive optical element functions as a lens.
According to the invention, optical systems of varied kinds, such as a photo-taking optical system and an observation optical system, can be arranged to include the above-stated diffractive optical elements which can be manufactured without difficulty.
The above and other objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.